You can also add things later on (items and pay for it) in the pledgemanager later. However that money doesn't count to the total for the project and stretch goals. So anything you think you may want to add later it is better to go ahead and add the money now for it. Including maybe even a little extra for the anticipated shipping, although hopefully there will be more definitive details about that later.
To my knowledge MCG hasn't yet confirmed or denied this. In Recon, they let PM funds count towards stretch goals (and made another 200k that way). In the first Myth KS they did not.